Print your own:

These invites would work well for lots of different types of summer events. If it is a more formal type of party, then use good quality paper for printing - it makes a big difference. Print out a few on samples to see which you like best.

Summer parties are great outside, but have a contingency plan in case the weather isn't as nice as you hoped.

There are lots of ideas for do-it-yourself decorations on my pinterest board. Why not take a look and see if it inspires you. Just small touches can sometimes make a world of difference to the atmosphere of your event - so plan carefully and feel about what you want to create.